Download Atomic Theory

Atomic Structure
• ATOM: the smallest particle that has the
properties of an element.
• From the early Greek concept of the atom
to the modern atomic theory, scientists
have built on and modified existing models
of the atom.
• Atoms are composed of a positively charged
nucleus surrounded by an electron cloud.
– Nucleus (99% of atom’s mass): uncharged
neutrons and positively charged protons.
– Electron cloud: negatively charged electrons in
constant motion creating a “cloud” like a fan.
DEMOCRITUS
• In 400 B.C., this Greek
philosopher suggested
that the universe was
made of indivisible units.
• “Atom” – Greek word
meaning “unable to be
divided”
JOHN DALTON
In 1808, this English
schoolteacher proposed his
own atomic theory that
became widely accepted.
Dalton’s Atomic Theory:
–
–
–
–
All elements are made of tiny atoms.
Atoms cannot be subdivided.
Atoms of the same element are exactly alike.
Atoms of different elements can join to form
molecules.
• As it turns out, the atom can be divided into
subatomic particles.
• Thompson and Millikan are given credit for the
first discoveries relating to electrons.
• Rutherford discovered the positively charged
nucleus
Rutherford Scattering Experiment
Rutherford Scattering Experiment
Rutherford Scattering Experiment
“I remember Geiger coming to me in great excitement
and saying, “We have been able to get some of the alphaparticles coming backwards.” It was quite the most incredible
event that has ever happened to me in my life. It was almost as
incredible as if you fired a 15 inch shell at a piece of paper and
it came back and hit you.”
Ernest Rutherford
Rutherford Scattering Experiment
Rutherford proposes the nucleus, and the “solar system” model.
NIELS BOHR
• In 1913, this Danish scientist
suggested that electrons “orbit” the
nucleus.
• In Bohr’s model, electrons are placed
in different energy levels based on
their distance from the nucleus.
Electrons are like books in a book case.
They can only exist on certain levels.
Only certain orbits are allowed
Only certain orbits are allowed
An electron
jumping up
absorbs light
Only certain orbits are allowed
An electron
jumping down
gives off light
Orbits – Energy Levels
n=4
n=3
n=2
n=1
Orbits – Energy Levels
n=4
n=3
n=2
n=1
Orbits – Energy Levels
n=4
n=3
n=2
n=1
Lyman Series (Ultraviolet)
Orbits – Energy Levels
n=4
n=3
n=2
Balmer Series (Visible)
n=1
Rydberg Equation
 1
1 
frequency  3.287 10  2  2 
 nTo nFrom 
15
Bohr Model of the Atom
Successful:
1. Explained the existence of line spectra.
2. Explained the frequencies of line spectra.
The Bohr Atom was a “Solar
System” model.
Bohr Model of the Atom
Defects:
1. Contradicted known physics.
2. Couldn’t explain various intensities of
the line spectra.
3. Couldn’t explain why only certain
orbits were allowed.
4. Only worked for hydrogen.
Physicists were mystified, but intrigued by
Bohr’s theory of the atom.
Why are the energies of the
hydrogen electron quantized?
Why is the electron restricted to
orbiting at certain fixed distances?
Louis de Broglie
For a decade no one,
not even Bohr himself had a
logical explanation.
In 1924 Louis de
Broglie provided a solution.
In his doctoral dissertation he
proposed that matter and radiation
have both wave and particle
properties.
He won the nobel prize in 1929
If electrons are waves, what is it that’s
waving?
Einstein proposes it’s the probability
of finding an electron at any given
place.
MODERN ATOMIC
MODEL
• By 1925, Bohr’s model of the atom no longer
explained all observations. Bohr was
correct about energy levels, but wrong
about electron movement.
• Electrons occupy the lowest energy levels
available.
• Energy increases as distance from the
nucleus increases.
• Electrons move in patterns of “wave
functions” around the nucleus.
• It is impossible to know an electrons
velocity and location at any moment in time.
Schrödinger
This is very
difficult stuff!
In 1926 Schrodinger proposes
an equation that gives the
probability of finding an
Don’t worry, this
electron at any place in the won’t be on the
test.
atom.
  


 V  i
2
2m x
t
2
2
ORBITALS
• ORBITAL: the regions in an atom where there is a
high probability of finding electrons.
• s is the lowest energy orbital, and p is slightly
higher
• d and f are the next two orbitals. They occupy
even higher energy levels and take on more
complex shapes than s & p
VALENCE ELECTRONS
• Electrons in the outermost energy level are
called VALENCE ELECTRONS.
• Valence electrons determine how an atom
will act in a chemical reaction.
• Atoms with equal numbers of valence
electrons have similar properties.
• We will learn how to determine the # of
valence electrons in an atom later in this
unit.
What part of the atom is much smaller than
the atom, yet contains most of the mass?
a) the nucleus
b) the electron cloud
Not drawn to scale
While its diameter is very small compared to
that of the entire atom, 99% of the mass of an
atom comes from the protons and neutrons in
the nucleus.
Which statement is true according to Dalton’s
atomic theory?
a)
b)
c)
d)
Atoms of different elements join to form larger
atoms.
Atoms can be subdivided into smaller particles.
Atoms of the same element differ in electric
charge.
Atoms of the same element are exactly alike.
This statement was part of Dalton’s atomic
theory. While this statement was holds true if
we remove the word “exactly”, we have since
found that atoms of an element often differ in
number of neutrons (isotopes).
According to Bohr’s model of the atom,
electrons behave like
a)
b)
c)
d)
planets rotating on their axes.
planets orbiting the sun.
light energy in a vacuum.
waves vibrating on a string.
While Bohr’s model of energy levels of
electrons around an atom remains supported,
his model of electron movement has been
revised. Schrödinger and other scientists
have since found that electrons travel in
wave-like patterns around the nucleus.
Schrödinger
1869: created first periodic table of elements.
Arranged elements in order of increasing atomic
mass.
One of Rutherford’s students.
1914: Arranged the elements in order of increasing
atomic number (responsible for TODAY’S periodic
table).
Organization of the Periodic Table
• PERIODICITY: regular variations (or patterns)
of properties with increasing atomic number.
Both chemical and physical properties vary in a
periodic (repeating) pattern.
• PERIOD: horizontal row of elements on P.T.
• GROUP (FAMILY): vertical column of elements
on P.T.
Periodic Key
6
C
Atomic number (Z)
Element’s symbol
Carbon
Element’s name
12.011
Atomic mass (A)
# of protons = Z
# of electrons = # of protons (in a neutral atom)
# of neutrons = A-Z
ISOTOPES
• Isotopes are atoms that have the same #
of protons, but a different # of neutrons.
• Example:
Carbon-12 vs. Carbon-14
12C
Mass # = 12; Atomic # = 6 (6P, 6E, 6N)
14C
Mass # = 14; Atomic # = 6 (6P, 6E, 8N)
IONS
• Ionization: the process of adding or
removing electrons from an atom or
group of atoms.
• An ion has a net electric charge.
• Cation: ion with a positive charge.
Ex: Na+
• Anion: ion with a negative charge.
Ex: O2-
Valence electrons &
electron dot diagrams
• Review: The valence electrons are the
outermost electrons in an atom. These are
the electrons that are involved when there
is a chemical reaction.
• Looking at a periodic table, you can quickly
determine how many valence electrons an
atom has by what column it is in.
# of Valence electrons
2
1
2
3
4
5
6 7 8
ELECTRON DOT DIAGRAMS:
visual representations of elements and their valence electrons
Standard form:
36
4
7
Order of electron/dot
placement
R
58
1
2
element symbol
Example:
Oxygen
O
6 valence
electrons
Determining # P+, N, and Efrom chemical symbols:
Mass #
• Example 1: atom
# protons = 6
#electrons= 6
#neutrons= 14-6 = 8
No net
charge
14
6
C
Atomic #
• Example 2: ion
# protons = 7
#electrons= 10
#neutrons= 15-7 = 8
15
7
N3-
Net
charge
of -3
The periodic law states that when elements
are arranged in order of increasing
__________ _________, similarities in their
properties occur in a regular pattern.
a) Atomic mass
b) Atomic number
c) Atomic radius
Moseley created the modern periodic
table when he determined that elements
should be placed in order of increasing
atomic number (# of protons). It’s a
shame that WWI took the life of this
brilliant scientist when he was so young.
Carbon-12 and carbon-14 are
a)
b)
c)
d)
isomers
isotopes
radioactive elements
different elements
Isotopes are atoms of the same element with
different numbers of neutrons, and therefore
different atomic masses.
While carbon-14 is used in radioactive dating,
carbon-12 has a more stable nucleus and therefore
is not used in this capacity.
How many protons, electrons and
neutrons are in one atom of oxygen-17?
a)
b)
c)
d)
17 p, 17 e, 17 n
17 p, 17 e, 1 n
8 p, 8 e, 8 n
8 p, 8 e, 9 n
While most oxygen atoms have a mass of 16 g/mol,
oxygen-17 is an isotope with a mass of 17 g/mol.
The number of protons in an element is the same
for every atom of that element.
# electrons = # protons if element has no charge.
How many protons, electrons and
neutrons are in O2-?
a)
b)
c)
d)
10 p, 8 e, 8 n
8 p, 8 e, 8 n
8 p, 10 e, 8 n
8 p, 6 e, 8 n
Every atom or ion of oxygen has 8 protons.
The net charge of -2 indicates that the ion has two more
negative charges (electrons) than positive charges (protons)
All atoms and ions of standard oxygen-16 have 8 neutrons.
How many dots are in an electron dot
diagram of chlorine?
a)
b)
c)
d)
1
7
17
35
Chlorine and all of the other
halogens (column 17) have
seven valence electrons.
Cl